Synthesis of WS42- Intercalated NiZnAl LDHs as Effective Adsorbents to Remove Copper Ions from Water.

Abstract

A series of WS42- intercalated NiZnAl ternary-layered double-hydroxides (LDHs) with various Ni/Zn ratios were synthesized by an ion-exchange method and used as adsorbents to remove Cu2+ from water. The introduction of Zn produced ZnS on the surface of LDHs. The LDH with the Ni/Zn/Al molar ratio of 0.1/1.9/1 showed the best adsorption ability. Cu2+ ions are removed via three routes: forming [Cu-WS4]n- complexes via soft acid-soft base interaction between WS42- and Cu2+, isomorphic substitution of Zn2+ in sheets by Cu2+, and cation exchange of Cu2+, with ZnS on the surface of LDHs. With the increased Cu2+ concentration, the complexes dominated the adsorption because polynuclear [Cu-WS4]n- complexes with high Cu/W ratios (2-6) may be formed. Cu+ is present in such complexes, which is produced by the internal redox. Even at Cu2+ concentration up to 600 mg·L-1, neither amorphous CuWS4 nor decreased interlayer distance was observed. Contrarily, the interlayer distance was slightly enlarged due to forming bigger [Cu-WS4]n- complexes. The adsorption followed the pseudo-second-order kinetics and Langmuir isotherm model. The experimental maximum adsorption capacity reached 555.4 mg·g-1.